The cultured human lymphocyte (IM-9) binds 125I-insulin by a receptor-mediated process; the receptor, in turn, is regulated by the ligand. In the present study we have examined quantitatively the morphologic events involved in 125I-insulin interaction with the surface of the lymphocyte. At 2 min of incubation of 15 degrees or 37 degrees C, the ligand localizes preferentially at the villous surface of the cell, whereas with longer periods of incubation, the ligand distributes indistinguishably between the villous and nonvillous surface. When rebinding is blocked, 125I-insulin localizes preferentially at the nonvillous surface of the cell. When the total cell surface is considered, there is little preferential association with coated pits; when only the nonvillous surface is considered, a preferential association with coated pits is found and is quantitatively increased in the absence of rebinding of the ligand. This cell has an abundant villous surface (approximately 55% of the total surface); and, as seen on freeze-fracture replicas, the plasma membrane of the villous surface contains a 60% greater density of intramembrane particles than the nonvillous surface. The data suggest an ordered pattern of insulin interaction with the cell surface (i.e., binding to villi followed by redistribution to the nonvillous portion of the cell containing coated pits). These events probably reflect the mechanism by which the cell segregates specific receptors and related proteins in the plane of the membrane so that they can be selectively removed.

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